1. Field of the Invention
The invention generally relates to a coaxial-based telephony system, and more particularly to methods and apparatus for protection switching among different frequency bands or different units of equipment in a frequency division multiplexed communication system.
2. Description of Related Art
Co-axial cable systems have been widely deployed for providing video signals, such as cable television (CATV) signals, to subscriber locations, i.e. homes or offices. Several such systems also provide telephony signals over the co-axial cables, such as signals carrying telephone calls, facsimile transmissions, Internet data communications and the like.
With systems for providing both video and telephony signals over a single co-axial cable, the single cable thereby carries both downstream signals (i.e. signals sent to the subscriber location) and upstream signals (i.e. signals sent from the subscriber location). The downstream signals include both video and telephony signals. The upstream signals typically include only telephony. In some systems, though, upstream signals additionally include upstream video signals such as may be required with interactive television systems.
Separate transmission frequencies typically are used to distinguish the downstream signals from the upstream signals, to distinguish downstream telephony signals from downstream video signals and to distinguish upstream telephony from upstream video signals, if any. Moreover, as to the telephony signals, otherwise conventional pair gain techniques may be employed to permit simultaneous transmission of two or more telephony channels both upstream and downstream to thereby permit, for example, two separate telephone conversations to proceed simultaneously using two separate telephones at the subscriber location. The signals carried on the telephony channels are typically encoded digitally for transmission using, for example, T1 framing.
When implementing such a combined video/telephony co-axial cable transmission system, a co-axial termination unit (CTU) (also sometimes referred to herein as a remote unit) may be provided at each individual subscriber location, with hundreds or perhaps thousands of CTU""s connected to a single combined video/telephony co-axial cable. Each CTU is connected to the combined video/telephony co-axial cable via a tap. Each CTU is also connected both to the upstream end of a video-only co-axial cable connected into the subscriber location, and to the upstream ends of any telephone circuits that are also connected into the subscriber location. The video-only co-axial cable is typically connected to a television set, cable TV decoder unit, or video cassette recorder (VCR) at the subscriber location. Tip and ring lines of the telephone circuits are typically connected to a telephone, facsimile machine or modem at the subscriber location.
Thus the CTU provides an interface between the combined video/telephony co-axial cable and the video-only co-axial cable and separate telephone lines connected into a single subscriber location. To this end, the CTU includes components for converting radio frequency (RF) digital telephony signals received on the telephony channels of the combined video/telephony co-axial cable to analog telephone signals for coupling to the tip and ring lines of the subscriber telephone circuits. Likewise, the CTU includes components for converting analog signals received from the tip and ring circuits to digital signals modulated onto an RF carrier for transmitting over the combined video/telephony co-axial cable. A modem and a coder-decoder (CODEC) may be employed to handle the conversions for the telephony operations. Also the CTU includes circuitry for routing the video signals received from the combined video/telephony co-axial cable to the video-only co-axial cable routed into the subscriber location. The video-only co-axial cable is referred to herein as a xe2x80x9cvideo-onlyxe2x80x9d cable only because, in use, it carries only video signals. The video-only co-axial cable is, however, an otherwise standard co-axial cable which could carry other signals as well.
An upstream end of the combined video/telephony co-axial cable is connected via an appropriate interface system into a telephone company central office (CO) provided with switching equipment for routing telephony signals to and from the public switched telephone network (PSTN). The interface system receives telephony signals from the PSTN via the CO and also receives video signals from a suitable video source, such as a CATV service provider or a satellite dish, and combines those signals onto the combined video/telephony co-axial cable for transmission to the CTU.
In one architecture, the frequency range of 450 to 750 MHz is employed for downstream signals and the frequency range of 5 to 50 MHz is employed for upstream signals. The upstream telephony frequency range is divided up into a plurality of smaller frequency bands, each having a bandwidth of, for example, 2 MHz. The downstream telephony frequency range is divided up in a similar manner. Each 2 MHZ frequency band is further multiplexed (xe2x80x9csub-multiplexedxe2x80x9d), such as by Discrete multitone (DMT) or Discrete Wavelet Multitone (DWMT) technology, so as to carry a plurality of communication channels (sometimes referred to herein as xe2x80x9csub-channelsxe2x80x9d). DMT and DWMT are multiplexing techniques which split bandwidth usage into sub-channels for maximum data transfer. DMT is described in J. S. Chow, J. C. Tu, and J. M. Cioffi, xe2x80x9cA discrete multitone transceiver system for HDSL applications,xe2x80x9d IEEE Journal on Selected Areas in Communications, vol. 9, no. 6, pp. 257-266 (1993), incorporated herein by reference, and DWMT is described in Richard Gross, Michael Tzannes, Stuart Sandberg, Halil Padir, and Xuming Zhang, xe2x80x9cDiscrete Wavelet Multitone (DWMT) System for Digital Transmission over HFC Linksxe2x80x9d, SPIE Proceedings, Volume 2609 (1995), incorporated herein by reference. A channel is then optimized for modulation if certain sub-channels cannot transmit data due to noise, for example. Noise problems are inherent in coaxial cable telephony in both the upstream and downstream directions, but are most acute in the upstream direction, for example, as the result of the presence of noise sources within the 5 to 50 MHz band (such as motors, washers, compressors and the like) operating near the downstream end of the co-axial cable.
An important consideration in any telephony system is the mechanism that the system uses to accommodate and avoid faulty equipment or equipment providing an unacceptably poor level of quality. Most systems implement a technique known as protection switching, in which a fault or unacceptable quality is detected and the service or services that are affected are moved, either manually or automatically, to other equipment which is providing service of sufficient quality. In the case of an RF system, quality degradation might derive not only from equipment problems, but also from external RF noise sources as mentioned above. Thus in the case of an RF system, it may be possible to remedy a quality degradation problem merely by moving an affected service to a different sub-multiplexed channel within the same frequency band. This kind of protection switching, referred to herein as intra-band or intra-channel protection switching, can be least disruptive to services in progress (such as continuing telephone calls) if the intra-band protection switching does not require any handoff of the service from one unit of equipment to another. The latter condition might exist, for example, if all the sub-channels in each 2 MHZ band are served by common component equipment (such as a common modem), but different 2 MHZ bands are served by different equipment.
If intra-band protection switching is not successful, then an RF system would next try moving the affected service(s) to a different frequency band. This kind of protection switching is referred to herein as inter-band or inter-channel protection switching. Intra-band and inter-band protection switching are also sometimes referred to herein as intra-modem and inter-modem protection switching, respectively, specifically because the embodiment described hereinafter assigns each 2 MHZ channel in a given direction to a different modem.
In the past, if inter-band protection switching was required, it would have been assumed that the entire 2 MHZ frequency band (or some unit of equipment serving the entire 2 MHZ band) was bad, and all services being carried within that frequency band would have been moved to a different band. Stated another way, in an architecture in which each 2 MHZ frequency band is served by different units of equipment, it would have been assumed that all remote units that had been served by one unit of equipment would have to be moved to another unit of equipment. One spare unit of equipment would have been provided for this purpose. This approach to inter-band protection switching involves what is known as xe2x80x9c1xc3x97N redundancyxe2x80x9d because one spare unit of equipment is available to protect N active units of equipment.
One problem with the 1xc3x97N approach is that maintenance is typically required in response to each inter-band protection switching event in order to isolate the problem and correct it. Otherwise, the group of N units of equipment will be left without any inter-band protection switching capability. If another inter-band protection switching event is later required and no spare equipment units are available, the affected service(s) would likely be dropped. This can be upsetting to customers, for example, who might find their telephone call suddenly disconnected.
Another problem with the 1xc3x97N redundancy approach especially in RF communication systems, is that sometimes the first protection switching event is caused by RF interference which degrades more than one 2 MHZ-wide frequency band simultaneously. In this situation only one inter-band protection switching event would be accommodated; any further inter-band protection switching required due to the same interference would result in dropped calls. Of course both of these problems could be alleviated by providing two (or more) spare units of equipment and a corresponding number of spare frequency bands per N frequency bands in use, but this solution would provide only incrementally improved protection at twice (or more) the cost.
Accordingly it would be desirable to provide an inter-band protection switching technique which provides greater redundancy than 1xc3x97N, without substantially greater cost.
In accordance with the invention, roughly described, a system is provided which implements what is essentially xe2x80x9cMxc3x97N redundancy.xe2x80x9d That is, a service (e.g. a call) which needs to be moved to a different channel can be moved without having to move other services making use of the same channel. In an embodiment in which different channels are controlled by different units of equipment, the invention can be viewed as a technique in which when a service using a channel controlled by one unit of equipment needs to be moved to a different unit of equipment, fewer than all the other services using the first unit of equipment are moved to the second unit of equipment. It is not necessary to have a pre-assigned xe2x80x9csparexe2x80x9d unit of equipment available for redundancy purposes, although that still would be possible within the inventive scheme. Instead, the redundancy scheme described herein can take advantage of spare bandwidth (in the form of spare sub-multiplexed channels) on other active units of equipment.
In one embodiment, in which telephony is carried on sub-multiplexed channels within multiplexed channels carried on a combined video/telephony co-axial cable transmission system, inter-band protection switching can take place under the control of a Terminal Control Processor (TCP) at the head end in response to messages from the equipment units responsible for respective telephony channels. In order to optimize the use of intra-band protection switching, the TCP manages assignment of coax remote units to head-end equipment units in accordance with certain leveling rules. First, the number of DS0""s available for protection on any given CLUH is maximized. Second, the level of concentration on any given CLUH is minimized. Third, the number of remote units communicating with any one CLUH is minimized, and is not to exceed a predetermined number (e.g. 128).
Other aspects of the invention as well as other advantages of the invention are provided as well. Method embodiments of the invention are also provided.